jq can transform JSON in various ways, by selecting, iterating, reducing
and otherwise mangling JSON documents. For instance, running the command jq
´map(.price) | add´ will take an array of JSON objects as
input and return the sum of their "price" fields.

jq can accept text input as well, but by default, jq reads a
stream of JSON entities (including numbers and other literals) from
stdin. Whitespace is only needed to separate entities such as 1 and 2,
and true and false. One or more files may be specified, in which case
jq will read input from those instead.

The options are described in the INVOKING JQ section; they mostly
concern input and output formatting. The filter is written in the jq
language and specifies how to transform the input file or document.

A jq program is a "filter": it takes an input, and produces an output.
There are a lot of builtin filters for extracting a particular field of an
object, or converting a number to a string, or various other standard tasks.

Filters can be combined in various ways - you can pipe the output of one filter
into another filter, or collect the output of a filter into an array.

Some filters produce multiple results, for instance there´s one that
produces all the elements of its input array. Piping that filter into a second
runs the second filter for each element of the array. Generally, things that
would be done with loops and iteration in other languages are just done by
gluing filters together in jq.

It´s important to remember that every filter has an input and an output.
Even literals like "hello" or 42 are filters - they take an input
but always produce the same literal as output. Operations that combine two
filters, like addition, generally feed the same input to both and combine the
results. So, you can implement an averaging filter as add / length -
feeding the input array both to the add filter and the length
filter and then performing the division.

jq filters run on a stream of JSON data. The input to jq is parsed as a sequence
of whitespace-separated JSON values which are passed through the provided
filter one at a time. The output(s) of the filter are written to standard out,
again as a sequence of whitespace-separated JSON data.

Note: it is important to mind the shell´s quoting rules. As a general
rule it´s best to always quote (with single-quote characters) the jq
program, as too many characters with special meaning to jq are also shell
meta-characters. For example, jq "foo" will fail on most Unix
shells because that will be the same as jq foo, which will generally
fail because foo is not defined. When using the Windows command shell
(cmd.exe) it´s best to use double quotes around your jq program when
given on the command-line (instead of the -f program-file option), but
then double-quotes in the jq program need backslash escaping.

You can affect how jq reads and writes its input and output using some
command-line options:

•

--version:

Output the jq version and exit with zero.

•

--seq:

Use the application/json-seq MIME type scheme for
separating JSON texts in jq´s input and output. This means that an
ASCII RS (record separator) character is printed before each value on
output and an ASCII LF (line feed) is printed after every output. Input
JSON texts that fail to parse are ignored (but warned about), discarding
all subsequent input until the next RS. This more also parses the output
of jq without the --seq option.

This is useful for processing very large inputs. Use this
in conjunction with filtering and the reduce and foreach
syntax to reduce large inputs incrementally.

•

--slurp/-s:

Instead of running the filter for each JSON object in the
input, read the entire input stream into a large array and run the filter
just once.

•

--raw-input/-R:

Don´t parse the input as JSON. Instead, each line of
text is passed to the filter as a string. If combined with --slurp,
then the entire input is passed to the filter as a single long
string.

•

--null-input/-n:

Don´t read any input at all! Instead, the filter is
run once using null as the input. This is useful when using jq as a
simple calculator or to construct JSON data from scratch.

•

--compact-output / -c:

By default, jq pretty-prints JSON output. Using this option
will result in more compact output by instead putting each JSON object on
a single line.

•

--tab:

Use a tab for each indentation level instead of two
spaces.

•

--indent n:

Use the given number of spaces (no more than 8) for
indentation.

•

--color-output / -C and
--monochrome-output / -M:

By default, jq outputs colored JSON if writing to a
terminal. You can force it to produce color even if writing to a pipe or a
file using -C, and disable color with -M.

•

--ascii-output / -a:

jq usually outputs non-ASCII Unicode codepoints as UTF-8,
even if the input specified them as escape sequences (like
"\u03bc"). Using this option, you can force jq to produce pure
ASCII output with every non-ASCII character replaced with the equivalent
escape sequence.

•

--unbuffered

Flush the output after each JSON object is printed (useful
if you´re piping a slow data source into jq and piping jq´s
output elsewhere).

•

--sort-keys / -S:

Output the fields of each object with the keys in sorted
order.

•

--raw-output / -r:

With this option, if the filter´s result is a string
then it will be written directly to standard output rather than being
formatted as a JSON string with quotes. This can be useful for making jq
filters talk to non-JSON-based systems.

•

--join-output / -j:

Like -r but jq won´t print a newline after
each output.

•

-f filename / --from-file filename:

Read filter from the file rather than from a command line,
like awk´s -f option. You can also use ´#´ to make
comments.

•

-Ldirectory / -L directory:

Prepend directory to the search list for modules. If
this option is used then no builtin search list is used. See the section
on modules below.

•

-e / --exit-status:

Sets the exit status of jq to 0 if the last output values
was neither false nor null, 1 if the last output value was
either false or null, or 4 if no valid result was ever
produced. Normally jq exits with 2 if there was any usage problem or
system error, 3 if there was a jq program compile error, or 0 if the jq
program ran.

•

--arg name value:

This option passes a value to the jq program as a
predefined variable. If you run jq with --arg foo bar, then
$foo is available in the program and has the value
"bar". Note that value will be treated as a
string, so --arg foo 123 will bind $foo to
"123".

•

--argjson name JSON-text:

This option passes a JSON-encoded value to the jq program
as a predefined variable. If you run jq with --argjson foo 123,
then $foo is available in the program and has the value
123.

•

--slurpfile variable-name filename:

This option reads all the JSON texts in the named file and
binds an array of the parsed JSON values to the given global variable. If
you run jq with --argfile foo bar, then $foo is available in
the program and has an array whose elements correspond to the texts in the
file named bar.

•

--argfile variable-name filename:

Do not use. Use --slurpfile instead.

(This option is like --slurpfile, but when the file
has just one text, then that is used, else an array of texts is used as in
--slurpfile.)

•

--run-tests [filename]:

Runs the tests in the given file or standard input. This
must be the last option given and does not honor all preceding options.
The input consists of comment lines, empty lines, and program lines
followed by one input line, as many lines of output as are expected (one
per output), and a terminating empty line. Compilation failure tests start
with a line containing only "%%FAIL", then a line containing the
program to compile, then a line containing an error message to compare to
the actual.

You can also look up fields of an object using syntax like
.["foo"] (.foo above is a shorthand version of this). This
one works for arrays as well, if the key is an integer. Arrays are zero-based
(like javascript), so .[2] returns the third element of the array.

The .[10:15] syntax can be used to return a subarray of an array or
substring of a string. The array returned by .[10:15] will be of length
5, containing the elements from index 10 (inclusive) to index 15 (exclusive).
Either index may be negative (in which case it counts backwards from the end
of the array), or omitted (in which case it refers to the start or end of the
array).

The .[2] syntax can be used to return the element at the given index.
Negative indices are allowed, with -1 referring to the last element, -2
referring to the next to last element, and so on.

The .foo syntax only works for simply keys i.e. keys that are all
alphanumeric characters. .[<string>] works with keys that contain
special characters such as colons and dots. For example
.["foo::bar"] and .["foo.bar"] work while
.foo::bar and .foo.bar would not.

The ? "operator" can also be used with the slice operator, as
in .[10:15]?, which outputs values where the inputs are slice-able.

If you use the .[index] syntax, but omit the index entirely, it will
return all of the elements of an array. Running .[] with the
input [1,2,3] will produce the numbers as three separate results,
rather than as a single array.

You can also use this on an object, and it will return all the values of the
object.

If two filters are separated by a comma, then the input will be fed into both
and there will be multiple outputs: first, all of the outputs produced by the
left expression, and then all of the outputs produced by the right. For
instance, filter .foo, .bar, produces both the "foo" fields
and "bar" fields as separate outputs.

The | operator combines two filters by feeding the output(s) of the one on the
left into the input of the one on the right. It´s pretty much the same
as the Unix shell´s pipe, if you´re used to that.

If the one on the left produces multiple results, the one on the right will be
run for each of those results. So, the expression .[] | .foo retrieves
the "foo" field of each element of the input array.

jq supports the same set of datatypes as JSON - numbers, strings, booleans,
arrays, objects (which in JSON-speak are hashes with only string keys), and
"null".

Booleans, null, strings and numbers are written the same way as in javascript.
Just like everything else in jq, these simple values take an input and produce
an output - 42 is a valid jq expression that takes an input, ignores
it, and returns 42 instead.

As in JSON, [] is used to construct arrays, as in [1,2,3]. The
elements of the arrays can be any jq expression. All of the results produced
by all of the expressions are collected into one big array. You can use it to
construct an array out of a known quantity of values (as in [.foo, .bar,
.baz]) or to "collect" all the results of a filter into an array
(as in [.items[].name])

Once you understand the "," operator, you can look at jq´s
array syntax in a different light: the expression [1,2,3] is not using
a built-in syntax for comma-separated arrays, but is instead applying the
[] operator (collect results) to the expression 1,2,3 (which produces
three different results).

If you have a filter X that produces four results, then the expression
[X] will produce a single result, an array of four elements.

If the keys are "sensible" (all alphabetic characters), then the
quotes can be left off. The value can be any expression (although you may need
to wrap it in parentheses if it´s a complicated one), which gets
applied to the {} expression´s input (remember, all filters have an
input and an output).

{foo: .bar}

will produce the JSON object {"foo": 42} if given the JSON
object {"bar":42, "baz":43}. You can use this to
select particular fields of an object: if the input is an object with
"user", "title", "id", and "content"
fields and you just want "user" and "title", you can write

{user: .user, title: .title}

Because that´s so common, there´s a shortcut syntax: {user,
title}.

If one of the expressions produces multiple results, multiple dictionaries will
be produced. If the input´s

Some jq operator (for instance, +) do different things depending on the
type of their arguments (arrays, numbers, etc.). However, jq never does
implicit type conversions. If you try to add a string to an object
you´ll get an error message and no result.

The operator + takes two filters, applies them both to the same input,
and adds the results together. What "adding" means depends on the
types involved:

•

Numbers are added by normal arithmetic.

•

Arrays are added by being concatenated into a larger
array.

•

Strings are added by being joined into a larger
string.

•

Objects are added by merging, that is, inserting all
the key-value pairs from both objects into a single combined object. If
both objects contain a value for the same key, the object on the right of
the + wins. (For recursive merge use the * operator.)

null can be added to any value, and returns the other value unchanged.

These infix operators behave as expected when given two numbers. Division by
zero raises an error. x % y computes x modulo y.

Multiplying a string by a number produces the concatenation of that string that
many times. "x" * 0 produces null.

Dividing a string by another splits the first using the second as separators.

Multiplying two objects will merge them recursively: this works like addition
but if both objects contain a value for the same key, and the values are
objects, the two are merged with the same strategy.

The builtin function keys, when given an object, returns its keys in an
array.

The keys are sorted "alphabetically", by unicode codepoint order. This
is not an order that makes particular sense in any particular language, but
you can count on it being the same for any two objects with the same set of
keys, regardless of locale settings.

When keys is given an array, it returns the valid indices for that array:
the integers from 0 to length-1.

The keys_unsorted function is just like keys, but if the input is
an object then the keys will not be sorted, instead the keys will roughly be
in insertion order.

Outputs array representations of the given path expression in .. The
outputs are arrays of strings (keys in objects0 and/or numbers (array indices.

Path expressions are jq expressions like .a, but also .[]. There
are two types of path expressions: ones that can match exactly, and ones that
cannot. For example, .a.b.c is an exact match path expression, while
.a[].b is not.

path(exact_path_expression) will produce the array representation of the
path expression even if it does not exist in ., if . is
null or an array or an object.

path(pattern) will produce array representations of the paths matching
pattern if the paths exist in ..

Note that the path expressions are not different from normal expressions. The
expression path(..|select(type=="boolean")) outputs all the
paths to boolean values in ., and only those paths.

These functions convert between an object and an array of key-value pairs. If
to_entries is passed an object, then for each k: v entry in the
input, the output array includes {"key": k, "value":
v}.

from_entries does the opposite conversion, and with_entries(foo)
is a shorthand for to_entries | map(foo) | from_entries, useful for
doing some operation to all keys and values of an object. from_entries
accepts key, Key, Name, value and Value as keys.

The filter add takes as input an array, and produces as output the
elements of the array added together. This might mean summed, concatenated or
merged depending on the types of the elements of the input array - the rules
are the same as those for the + operator (described above).

The filter flatten takes as input an array of nested arrays, and produces
a flat array in which all arrays inside the original array have been
recursively replaced by their values. You can pass an argument to it to
specify how many levels of nesting to flatten.

The range function produces a range of numbers. range(4;10)
produces 6 numbers, from 4 (inclusive) to 10 (exclusive). The numbers are
produced as separate outputs. Use [range(4;10)] to get a range as an
array.

The one argument form generates numbers from 0 to the given number, with an
increment of 1.

The two argument form generates numbers from from to upto with an
increment of 1.

The three argument form generates numbers from to upto with an
increment of by.

Some arithmetic operations can yield infinities and "not a number"
(NaN) values. The isinfinite builtin returns true if its input
is infinite. The isnan builtin returns true if its input is a
NaN. The infinite builtin returns a positive infinite value. The
nan builtin returns a NaN. The isnormal builtin returns true if
its input is a normal number.

Note that division by zero raises an error.

Currently most arithmetic operations operating on infinities, NaNs, and
sub-normals do not raise errors.

group_by(.foo) takes as input an array, groups the elements having the
same .foo field into separate arrays, and produces all of these arrays
as elements of a larger array, sorted by the value of the .foo field.

Any jq expression, not just a field access, may be used in place of .foo.
The sorting order is the same as described in the sort function above.

The filter contains(b) will produce true if b is completely contained
within the input. A string B is contained in a string A if B is a substring of
A. An array B is contained in an array A if all elements in B are contained in
any element in A. An object B is contained in object A if all of the values in
B are contained in the value in A with the same key. All other types are
assumed to be contained in each other if they are equal.

The while(cond; update) function allows you to repeatedly apply an update
to . until cond is false.

Note that while(cond; update) is internally defined as a recursive jq
function. Recursive calls within while will not consume additional
memory if update produces at most one output for each input. See
advanced topics below.

The until(cond; next) function allows you to repeatedly apply the
expression next, initially to . then to its own output, until
cond is true. For example, this can be used to implement a factorial
function (see below).

Note that until(cond; next) is internally defined as a recursive jq
function. Recursive calls within until() will not consume additional
memory if next produces at most one output for each input. See advanced
topics below.

Now suppose you want to extract all of the filenames present. You need to
retrieve .name, .children[].name,
.children[].children[].name, and so on. You can do this with:

recurse(.children[]) | .name

When called without an argument, recurse is equivalent to
recurse(.[]?).

recurse(f) is identical to recurse(f; . != null) and can be used
without concerns about recursion depth.

recurse(f; condition) is a generator which begins by emitting . and then
emits in turn .|f, .|f|f, .|f|f|f, ... so long as the computed value satisfies
the condition. For example, to generate all the integers, at least in
principle, one could write recurse(.+1; true).

For legacy reasons, recurse_down exists as an alias to calling
recurse without arguments. This alias is considered deprecated
and will be removed in the next major release.

The recursive calls in recurse will not consume additional memory
whenever f produces at most a single output for each input.

Short-hand for recurse without arguments. This is intended to resemble
the XPath // operator. Note that ..a does not work; use
..|a instead. In the example below we use ..|.a? to find all the
values of object keys "a" in any object found "below"
..

bsearch(x) conducts a binary search for x in the input array. If the input is
sorted and contains x, then bsearch(x) will return its index in the array;
otherwise, if the array is sorted, it will return (-1 - ix) where ix is an
insertion point such that the array would still be sorted after the insertion
of x at ix. If the array is not sorted, bsearch(x) will return an integer that
is probably of no interest.

The @foo syntax is used to format and escape strings, which is useful for
building URLs, documents in a language like HTML or XML, and so forth.
@foo can be used as a filter on its own, the possible escapings are:

Applies percent-encoding, by mapping all reserved URI
characters to a %XX sequence.

@csv:

The input must be an array, and it is rendered as CSV with
double quotes for strings, and quotes escaped by repetition.

@tsv:

The input must be an array, and it is rendered as TSV
(tab-separated values). Each input array will be printed as a single line.
Fields are separated by a single tab (ascii 0x09). Input characters
line-feed (ascii 0x0a), carriage-return (ascii 0x0d), tab
(ascii 0x09) and backslash (ascii 0x5c) will be output as
escape sequences \n, \r, \t, \\
respectively.

@sh:

The input is escaped suitable for use in a command-line for
a POSIX shell. If the input is an array, the output will be a series of
space-separated strings.

@base64:

The input is converted to base64 as specified by RFC
4648.

This syntax can be combined with string interpolation in a useful way. You can
follow a @foo token with a string literal. The contents of the string
literal will not be escaped. However, all interpolations made inside
that string literal will be escaped. For instance,

@uri "https://www.google.com/search?q=\(.search)"

will produce the following output for the input
{"search":"what is jq?"}:

"https://www.google.com/search?q=what%20is%20jq%3F"

Note that the slashes, question mark, etc. in the URL are not escaped, as they
were part of the string literal.

jq provides some basic date handling functionality, with some high-level and
low-level builtins. In all cases these builtins deal exclusively with time in
UTC.

The fromdateiso8601 builtin parses datetimes in the ISO 8601 format to a
number of seconds since the Unix epoch (1970-01-01T00:00:00Z). The
todateiso8601 builtin does the inverse.

The fromdate builtin parses datetime strings. Currently fromdate
only supports ISO 8601 datetime strings, but in the future it will attempt to
parse datetime strings in more formats.

The todate builtin is an alias for todateiso8601.

The now builtin outputs the current time, in seconds since the Unix
epoch.

Low-level jq interfaces to the C-library time functions are also provided:
strptime, strftime, mktime, and gmtime. Refer to
your host operating system´s documentation for the format strings used
by strptime and strftime. Note: these are not necessarily stable
interfaces in jq, particularly as to their localization functionality.

The gmtime builtin consumes a number of seconds since the Unix epoch and
outputs a "broken down time" representation of time as an array of
numbers representing (in this order): the year, the month (zero-based), the
day of the month, the hour of the day, the minute of the hour, the second of
the minute, the day of the week, and the day of the year -- all one-based
unless otherwise stated.

The mktime builtin consumes "broken down time" representations
of time output by gmtime and strptime.

The strptime(fmt) builtin parses input strings matching the fmt
argument. The output is in the "broken down time" representation
consumed by gmtime and output by mktime.

The strftime(fmt) builtin formats a time with the given format.

The format strings for strptime and strftime are described in
typical C library documentation. The format string for ISO 8601 datetime is
"%Y-%m-%dT%H:%M:%SZ".

jq may not support some or all of this date functionality on some systems.

The expression ´a == b´ will produce ´true´ if the
result of a and b are equal (that is, if they represent equivalent JSON
documents) and ´false´ otherwise. In particular, strings are
never considered equal to numbers. If you´re coming from Javascript,
jq´s == is like Javascript´s === - considering values equal only
when they have the same type as well as the same value.

if A then B else C end will act the same as B if A produces
a value other than false or null, but act the same as C otherwise.

Checking for false or null is a simpler notion of "truthiness" than is
found in Javascript or Python, but it means that you´ll sometimes have
to be more explicit about the condition you want: you can´t test
whether, e.g. a string is empty using if .name then A else B end,
you´ll need something more like if (.name | length) > 0 then A
else B end instead.

If the condition A produces multiple results, it is considered "true"
if any of those results is not false or null. If it produces zero results,
it´s considered false.

jq supports the normal Boolean operators and/or/not. They have the same standard
of truth as if expressions - false and null are considered "false
values", and anything else is a "true value".

If an operand of one of these operators produces multiple results, the operator
itself will produce a result for each input.

not is in fact a builtin function rather than an operator, so it is
called as a filter to which things can be piped rather than with special
syntax, as in .foo and .bar | not.

These three only produce the values "true" and "false", and
so are only useful for genuine Boolean operations, rather than the common
Perl/Python/Ruby idiom of "value_that_may_be_null or default". If
you want to use this form of "or", picking between two values rather
than evaluating a condition, see the "//" operator below.

A filter of the form a // b produces the same results as a, if
a produces results other than false and null. Otherwise,
a // b produces the same results as b.

This is useful for providing defaults: .foo // 1 will evaluate to
1 if there´s no .foo element in the input. It´s
similar to how or is sometimes used in Python (jq´s or
operator is reserved for strictly Boolean operations).

Errors can be caught by using try EXP catch EXP. The first expression is
executed, and if it fails then the second is executed with the error message.
The output of the handler, if any, is output as if it had been the output of
the expression to try.

A convenient use of try/catch is to break out of control structures like
reduce, foreach, while, and so on.

For example:

# Repeat an expression until it raises "break" as an
# error, then stop repeating without re-raising the error.
# But if the error caught is not "break" then re-raise it.
try repeat(exp) catch .=="break" then empty else error;

jq has a syntax for named lexical labels to "break" or "go (back)
to":

label $out | ... break $out ...

The break $label_name expression will cause the program to to act as
though the nearest (to the left) label $label_name produced
empty.

The relationship between the break and corresponding label is
lexical: the label has to be "visible" from the break.

where: * STRING, REGEX and FLAGS are jq strings and subject to jq string
interpolation; * REGEX, after string interpolation, should be a valid PCRE
regex; * FILTER is one of test, match, or capture, as
described below.

Emit a stream of the non-overlapping substrings of the input that match the
regex in accordance with the flags, if any have been specified. If there is no
match, the stream is empty. To capture all the matches for each input string,
use the idiom [ expr ], e.g. [ scan(regex) ].

Emit the string obtained by replacing the first match of regex in the input
string with tostring, after interpolation. tostring should be a
jq string, and may contain references to named captures. The named captures
are, in effect, presented as a JSON object (as constructed by capture)
to tostring, so a reference to a captured variable named "x"
would take the form: "(.x)".

Variables are an absolute necessity in most programming languages, but
they´re relegated to an "advanced feature" in jq.

In most languages, variables are the only means of passing around data. If you
calculate a value, and you want to use it more than once, you´ll need
to store it in a variable. To pass a value to another part of the program,
you´ll need that part of the program to define a variable (as a
function parameter, object member, or whatever) in which to place the data.

It is also possible to define functions in jq, although this is is a feature
whose biggest use is defining jq´s standard library (many jq functions
such as map and find are in fact written in jq).

jq has reduction operators, which are very powerful but a bit tricky. Again,
these are mostly used internally, to define some useful bits of jq´s
standard library.

It may not be obvious at first, but jq is all about generators (yes, as often
found in other languages). Some utilities are provided to help deal with
generators.

Some minimal I/O support (besides reading JSON from standard input, and writing
JSON to standard output) is available.

In jq, all filters have an input and an output, so manual plumbing is not
necessary to pass a value from one part of a program to the next. Many
expressions, for instance a + b, pass their input to two distinct
subexpressions (here a and b are both passed the same input), so
variables aren´t usually necessary in order to use a value twice.

For instance, calculating the average value of an array of numbers requires a
few variables in most languages - at least one to hold the array, perhaps one
for each element or for a loop counter. In jq, it´s simply add /
length - the add expression is given the array and produces its
sum, and the length expression is given the array and produces its
length.

So, there´s generally a cleaner way to solve most problems in jq than
defining variables. Still, sometimes they do make things easier, so jq lets
you define variables using expression as $variable. All variable names
start with $. Here´s a slightly uglier version of the
array-averaging example:

length as $array_length | add / $array_length

We´ll need a more complicated problem to find a situation where using
variables actually makes our lives easier.

Suppose we have an array of blog posts, with "author" and
"title" fields, and another object which is used to map author
usernames to real names. Our input looks like:

We use a variable, $names, to store the realnames object, so that we can refer
to it later when looking up author usernames:

.realnames as $names | .posts[] | {title, author: $names[.author]}

The expression exp as $x | ... means: for each value of expression
exp, run the rest of the pipeline with the entire original input, and
with $x set to that value. Thus as functions as something of a
foreach loop.

Just as {foo} is a handy way of writing {foo: .foo}, so
{$foo} is a handy way of writing {foo:$foo}.

Multiple variables may be declared using a single as expression by
providing a pattern that matches the structure of the input (this is known as
"destructuring"):

. as {realnames: $names, posts: [$first, $second]} | ...

The variable declarations in array patterns (e.g., . as [$first,
$second]) bind to the elements of the array in from the element at index
zero on up, in order. When there is no value at the index for an array pattern
element, null is bound to that variable.

Variables are scoped over the rest of the expression that defines them, so

.realnames as $names | (.posts[] | {title, author: $names[.author]})

will work, but

(.realnames as $names | .posts[]) | {title, author: $names[.author]}

won´t.

For programming language theorists, it´s more accurate to say that jq
variables are lexically-scoped bindings. In particular there´s no way
to change the value of a binding; one can only setup a new binding with the
same name, but which will not be visible where the old one was.

From then on, increment is usable as a filter just like a builtin
function (in fact, this is how some of the builtins are defined). A function
may take arguments:

def map(f): [.[] | f];

Arguments are passed as filters, not as values. The same argument may be
referenced multiple times with different inputs (here f is run for each
element of the input array). Arguments to a function work more like callbacks
than like value arguments. This is important to understand. Consider:

def foo(f): f|f;
5|foo(.*2)

The result will be 20 because f is .*2, and during the first
invocation of f. will be 5, and the second time it will be 10
(5 * 2), so the result will be 20. Function arguments are filters, and filters
expect an input when invoked.

If you want the value-argument behaviour for defining simple functions, you can
just use a variable:

def addvalue(f): f as $f | map(. + $f);

Or use the short-hand:

def addvalue($f): ...;

With either definition, addvalue(.foo) will add the current
input´s .foo field to each element of the array.

Multiple definitions using the same function name are allowed. Each
re-definition replaces the previous one for the same number of function
arguments, but only for references from functions (or main program) subsequent
to the re-definition.

The reduce syntax in jq allows you to combine all of the results of an
expression by accumulating them into a single answer. As an example,
we´ll pass [3,2,1] to this expression:

reduce .[] as $item (0; . + $item)

For each result that .[] produces, . + $item is run to accumulate
a running total, starting from 0. In this example, .[] produces the
results 3, 2, and 1, so the effect is similar to running something like this:

The foreach syntax is similar to reduce, but intended to allow the
construction of limit and reducers that produce intermediate results
(see example).

The form is foreach EXP as $var (INIT; UPDATE; EXTRACT). Like
reduce, INIT is evaluated once to produce a state value, then
each output of EXP is bound to $var, UPDATE is evaluated
for each output of EXP with the current state and with $var
visible. Each value output by UPDATE replaces the previous state.
Finally, EXTRACT is evaluated for each new state to extract an output
of foreach.

This is mostly useful only for constructing reduce- and limit-like
functions. But it is much more general, as it allows for partial reductions
(see the example below).

As described above, recurse uses recursion, and any jq function can be
recursive. The while builtin is also implemented in terms of recursion.

Tail calls are optimized whenever the expression to the left of the recursive
call outputs its last value. In practice this means that the expression to the
left of the recursive call should not produce more than one output for each
input.

Some jq operators and functions are actually generators in that they can produce
zero, one, or more values for each input, just as one might expect in other
programming languages that have generators. For example, .[] generates
all the values in its input (which must be an array or an object), range(0;
10) generates the integers between 0 and 10, and so on.

Even the comma operator is a generator, generating first the values generated by
the expression to the left of the comma, then for each of those, the values
generate by the expression on the right of the comma.

The empty builtin is the generator that produces zero outputs. The
empty builtin backtracks to the preceding generator expression.

All jq functions can be generators just by using builtin generators. It is also
possible to define new generators using only recursion and the comma operator.
If the recursive call(s) is(are) "in tail position" then the
generator will be efficient. In the example below the recursive call by
_range to itself is in tail position. The example shows off three
advanced topics: tail recursion, generator construction, and sub-functions.

jq currently only has IEEE754 double-precision (64-bit) floating point number
support.

Besides simple arithmetic operators such as +, jq also has most standard
math functions from the C math library. C math functions that take a single
input argument (e.g., sin()) are available as zero-argument jq
functions. C math functions that take two input arguments (e.g., pow())
are available as two-argument jq functions that ignore ..

Availability of standard math functions depends on the availability of the
corresponding math functions in your operating system and C math library.
Unavailable math functions will be defined but will raise an error.

At this time jq has minimal support for I/O, mostly in the form of control over
when inputs are read. Two builtins functions are provided for this,
input and inputs, that read from the same sources (e.g.,
stdin, files named on the command-line) as jq itself. These two
builtins, and jq´s own reading actions, can be interleaved with each
other.

One builtin provides minimal output capabilities, debug. (Recall that a
jq program´s output values are always output as JSON texts on
stdout.) The debug builtin can have application-specific
behavior, such as for executables that use the libjq C API but aren´t
the jq executable itself.

Causes a debug message based on the input value to be produced. The jq
executable wraps the input value with ["DEBUG:",
<input-value>] and prints that and a newline on stderr, compactly.
This may change in the future.

With the --stream option jq can parse input texts in a streaming fashion,
allowing jq programs to start processing large JSON texts immediately rather
than after the parse completes. If you have a single JSON text that is 1GB in
size, streaming it will allow you to process it much more quickly.

However, streaming isn´t easy to deal with as the jq program will have
[<path>, <leaf-value>] (and a few other forms) as inputs.

Several builtins are provided to make handling streams easier.

The examples below use the the streamed form of [0,[1]], which is
[[0],0],[[1,0],1],[[1,0]],[[1]].

Streaming forms include [<path>, <leaf-value>] (to indicate
any scalar value, empty array, or empty object), and [<path>] (to
indicate the end of an array or object). Future versions of jq run with
--stream and -seq may output additional forms such as
["error message"] when an input text fails to parse.

Assignment works a little differently in jq than in most programming languages.
jq doesn´t distinguish between references to and copies of something -
two objects or arrays are either equal or not equal, without any further
notion of being "the same object" or "not the same
object".

If an object has two fields which are arrays, .foo and .bar, and
you append something to .foo, then .bar will not get bigger.
Even if you´ve just set .bar = .foo. If you´re used to
programming in languages like Python, Java, Ruby, Javascript, etc. then you
can think of it as though jq does a full deep copy of every object before it
does the assignment (for performance, it doesn´t actually do that, but
that´s the general idea).

All the assignment operators in jq have path expressions on the left-hand side.

The filter .foo = 1 will take as input an object and produce as output an
object with the "foo" field set to 1. There is no notion of
"modifying" or "changing" something in jq - all jq values
are immutable. For instance,

.foo = .bar | .foo.baz = 1

will not have the side-effect of setting .bar.baz to be set to 1, as the
similar-looking program in Javascript, Python, Ruby or other languages would.
Unlike these languages (but like Haskell and some other functional languages),
there is no notion of two arrays or objects being "the same array"
or "the same object". They can be equal, or not equal, but if we
change one of them in no circumstances will the other change behind our backs.

This means that it´s impossible to build circular values in jq (such as
an array whose first element is itself). This is quite intentional, and
ensures that anything a jq program can produce can be represented in JSON.

Note that the left-hand side of ´=´ refers to a value in ..
Thus $var.foo = 1 won´t work as expected ($var.foo is not
a valid or useful path expression in .); use $var | .foo = 1
instead.

If the right-hand side of ´=´ produces multiple values, then for
each such value jq will set the paths on the left-hand side to the value and
then it will output the modified .. For example,
(.a,.b)=range(2) outputs {"a":0,"b":0}, then
{"a":1,"b":1}. The "update" assignment
forms (see below) do not do this.

Note too that .a,.b=0 does not set .a and .b, but
(.a,.b)=0 sets both.

As well as the assignment operator ´=´, jq provides the
"update" operator ´|=´, which takes a filter on the
right-hand side and works out the new value for the property of . being
assigned to by running the old value through this expression. For instance,
.foo |= .+1 will build an object with the "foo" field set to the
input´s "foo" plus 1.

This example should show the difference between ´=´ and
´|=´:

Provide input ´{"a": {"b": 10}, "b":
20}´ to the programs:

.a = .b .a |= .b

The former will set the "a" field of the input to the "b"
field of the input, and produce the output {"a": 20}. The latter
will set the "a" field of the input to the "a"
field´s "b" field, producing {"a": 10}.

The left-hand side can be any general path expression; see path().

Note that the left-hand side of ´|=´ refers to a value in
.. Thus $var.foo |= . + 1 won´t work as expected
($var.foo is not a valid or useful path expression in .); use
$var | .foo |= . + 1 instead.

If the right-hand side outputs multiple values, only the last one will be used.

Lots more things are allowed on the left-hand side of a jq assignment than in
most languages. We´ve already seen simple field accesses on the left
hand side, and it´s no surprise that array accesses work just as well:

.posts[0].title = "JQ Manual"

What may come as a surprise is that the expression on the left may produce
multiple results, referring to different points in the input document:

.posts[].comments |= . + ["this is great"]

That example appends the string "this is great" to the
"comments" array of each post in the input (where the input is an
object with a field "posts" which is an array of posts).

When jq encounters an assignment like ´a = b´, it records the
"path" taken to select a part of the input document while executing
a. This path is then used to find which part of the input to change while
executing the assignment. Any filter may be used on the left-hand side of an
equals - whichever paths it selects from the input will be where the
assignment is performed.

This is a very powerful operation. Suppose we wanted to add a comment to blog
posts, using the same "blog" input above. This time, we only want to
comment on the posts written by "stedolan". We can find those posts
using the "select" function described earlier:

.posts[] | select(.author == "stedolan")

The paths provided by this operation point to each of the posts that
"stedolan" wrote, and we can comment on each of them in the same way
that we did before:

jq has a library/module system. Modules are files whose names end in .jq.

Modules imported by a program are searched for in a default search path (see
below). The import and include directives allow the importer to
alter this path.

Paths in the a search path are subject to various substitutions.

For paths starting with "~/", the user´s home directory is
substituted for "~".

For paths starting with "$ORIGIN/", the path of the jq executable is
substituted for "$ORIGIN".

For paths starting with "./" or paths that are ".", the path
of the including file is substituted for ".". For top-level programs
given on the command-line, the current directory is used.

Import directives can optionally specify a search path to which the default is
appended.

The default search path is the search path given to the -L command-line
option, else ["~/.jq", "$ORIGIN/../lib/jq",
"$ORIGIN/../lib"].

Null and empty string path elements terminate search path processing.

A dependency with relative path "foo/bar" would be searched for in
"foo/bar.jq" and "foo/bar/bar.jq" in the given search
path. This is intended to allow modules to be placed in a directory along
with, for example, version control files, README files, and so on, but also to
allow for single-file modules.

Consecutive components with the same name are not allowed to avoid ambiguities
(e.g., "foo/foo").

For example, with -L$HOME/.jq a module foo can be found in
$HOME/.jq/foo.jq and $HOME/.jq/foo/foo.jq.

Imports a module found at the given path relative to a directory in a search
path. A ".jq" suffix will be added to the relative path string. The
module´s symbols are prefixed with "NAME::".

The optional metadata must be a constant jq expression. It should be an object
with keys like "homepage" and so on. At this time jq only uses the
"search" key/value of the metadata. The metadata is also made
available to users via the modulemeta builtin.

The "search" key in the metadata, if present, should have a string or
array value (array of strings); this is the search path to be prefixed to the
top-level search path.

Imports a module found at the given path relative to a directory in a search
path as if it were included in place. A ".jq" suffix will be added
to the relative path string. The module´s symbols are imported into the
caller´s namespace as if the module´s content had been included
directly.

The optional metadata must be a constant jq expression. It should be an object
with keys like "homepage" and so on. At this time jq only uses the
"search" key/value of the metadata. The metadata is also made
available to users via the modulemeta builtin.

Imports a JSON file found at the given path relative to a directory in a search
path. A ".json" suffix will be added to the relative path string.
The file´s data will be available as $NAME::NAME.

The optional metadata must be a constant jq expression. It should be an object
with keys like "homepage" and so on. At this time jq only uses the
"search" key/value of the metadata. The metadata is also made
available to users via the modulemeta builtin.

The "search" key in the metadata, if present, should have a string or
array value (array of strings); this is the search path to be prefixed to the
top-level search path.

This directive is entirely optional. It´s not required for proper
operation. It serves only the purpose of providing metadata that can be read
with the modulemeta builtin.

The metadata must be a constant jq expression. It should be an object with keys
like "homepage". At this time jq doesn´t use this metadata,
but it is made available to users via the modulemeta builtin.